Drugs, such as amphetamine, cocaine, nicotine and morphine, which tend to be abused by humans often also serve as positive reinforcers in rats. These drug agent paradoxically produce a conditioned taste aversion (CTA) when paired with a flavored solution. Recent evidence suggests that CTAs based on such positively reinforcing agents may differ from CTAs based on drugs which are ineffective reinforcers. In fact, the term "aversion" may not correctly describe the response conditioned to a flavor paired with a positively reinforcing drug, even though the rat may avoid consuming that drug-paired flavored solution. When rats are exposed to a flavored solution paired with a drug which is not a reinforcing agent, they not only avoid consuming the flavored solution, but they also demonstrate distinctive orofacial and somatic rejection responses. However, when rats are exposed to a flavored solution paired with a drug which also serves as a positive reinforcer, they demonstrate an equally strong avoidance of the flavored solution, but they do not demonstrate rejection responses. The proposed experiments are divided into two sections. The experiments proposed in Section I will systematically assess the ability of a wide range of doses of various agents which serve as positive reinforcers or do not serve as positive reinforcers to condition taste avoidance, rejection responses and place preferences over conditioning trials. In Section II, the role of peripheral emetic mechanisms in the conditioning of-rejection responses will be determined. The contribution of the area postrema of the brainstem and the vagal afferents from the gut to the establishment of rejection CRs will be assessed. The aim of the experiments is to provide behavioral and physiological evidence that CTAs produced by positively reinforcing agents differ from CTAs produced by other drug agents.